The use of diffractive effects in optics and electro-magnetic and similar radiant energy systems is well known. Diffraction gratings have been known for many years and more recently holographic gratings have become available. Such gratings disperse the incident light according to its wavelength. Essentially all that is needed to do this is a regularly spaced array of slits. The width of each slit as a proportion of the opaque separation distance between adjacent slits has a critical effect on the optical action. The emergent light is diffracted so that as well as some of its energy continuing as before, a proportion is deviated to either side of the unaffected beam and at a series of angles or "orders" which relate to the spacing of the slits and the wavelength of the light. Such arrays of slits are known as amplitude gratings. Alternatively, the slit plus space distance can be made to allow the incident light to pass through but impose some phase difference over a proportion of this distance. As long as each pattern is regularly and accurately repeated over the whole grating this will allow the dispersive effects to build up. This type of grating is known as a "phase grating".
Diffractive lenses have received far less attention than diffraction gratings. A normal lens deviates light from a circle within its aperture by a constant angle to a point somewhere along the axis. The slits and spaces of a diffraction grating have therefore been made circles to achieve a form of diffractive lens.
However if one simply forms a series of annular slits or zones with slit and space widths of equal dimensions or with the slit and space dimensions imposing a phase difference, this will give the effect of producing a large number of focal powers and generate images for each of them. This is because, unlike the diffraction grating where the extra orders emerge at different angles, the extra orders in the case of the annular arrangement occur along its axis giving images at different focal distances. Such annular arrangements are known as Zone Plates. We have already described a different arrangement in GB 2 129 157 namely a diffractive lens which generates two images one by refraction and one by diffraction and can effectively be used as a bifocal artificial eye lens. We have now found that an additional advantageous form of diffractive lens is one which can correct for astigmatism by means of the diffractive effect by having the capability of diffracting light predominantly into one order at one orientation. Such a lens can be used as an artificial eye lens in the form of both intra-ocular lenses and contact lenses.
A diffractive lens, according to the present invention can be made to behave in the same manner as both a refractive cylindrical lens and a refractive lens with a toric surface. A toric surface is one which at every part of its surface has maximum and minimum curvatures orthogonal to one another. A spherical surface is in fact a special case of the toric surface where the maximum and minimum curvatures are equal. A cylindrical surface is one in which the minimum curvature is zero, and a flat surface is one where maximum and minimum curvatures are both zero.
In a refractive cylindrical lens, the axis of the cylinder, part of which forms the lens surface is parallel to the principal meridian which has zero curvature. This meridian is known as the cylinder axis. A refractive cylindrical lens with a vertical cylinder axis will produce a line image parallel to the cylinder axis. A toric refractive lens will form two line images. Thus if one has a refractive lens with a toric surface having powers for example +6D and +4D, one can achieve the same effect by using a +4D thin spherical lens in contact with a thin cylindrical lens of +2D power. Other ways of achieving the same effect are by using
(a) a +6D thin spherical lens in contact with a thin cylindrical lens of -2D power and (b) so called crossed cylinders by using two cylindrical lenses of powers +6D and +4D with their cylinder axes at right angles.
Diffractive lenses can be made which emulate the optical effect of a refractive cylindrical lens and of combining lenses in the manner just described so that astigmatism can be corrected by diffraction. Spherical power can be provided by normal refractive methods and lenses can also be made in which at least some of the spherical power of the lens is provided by means of rotationally symmetrical diffraction zones as disclosed in GB 2 129 157. It has further been found that a combination of the control of astigmatism by diffracting light into predominantly one order in one orientation, with the provision of spherical power in the manner disclosed in GB 2 129 157 in one lens enables presbyopes with astigmatism to wear contact lenses in which any astigmatism correction is also produced by diffraction.